1. Field of the Invention
The present invention relates to a gas-lift petroleum well for producing reservoir fluids which uses reservoir gas for production. In one aspect, the present invention relates to a system and method of using an electronically controllable downhole valve and downhole pressurized gas to lift fluids up a well for petroleum production purposes.
2. Description of the Related Art
Gas lift is widely used to generate artificial lift in oil wells having insufficient reservoir pressure to drive formation fluids to the surface. In current practice lift gas is supplied to the well by surface compressors connected through an injection control valve to an annular space formed between a production tubing and a well casing. The gas flows down the annular space to a downhole gas-lift valve, which fluidly connects the annular space to the interior of the tubing. The gas-lift valve may be located just above the oil production zone, and the lift is generated by the combination of reduced density in the fluid column filling the tubing caused by gas bubbles from the gas-lift valve, and by entrained flow of the fluids by the rising gas stream in the tubing.
A variety of flow regimes in the tubing are recognized, and are determined by the gas flow rate at the gas-lift valve. The gas bubbles in the tubing decompress as they rise in the tubing because the head pressure of the fluid column above drops as the bubbles rise. This decompression causes the bubbles to expand, so that the flow regimes within the tubing can vary up the tubing, depending on the volumetric ratio of bubbles to liquid. Other factors contribute to determining the flow regime, such as fluid column height, fluid composition and phases present, tubing diameter, depth of well, temperature, back pressure set by the production control valve, and physical characteristics of the surface collection system. For the effective use of gas lift, it is important to control the injection rate of the lift gas.
Conventionally, the injection rate at the gas-lift valve is determined by the pressure difference across the valve, and its orifice size. In existing practice, the pressure on the annulus side is determined by the gas supply flow rate at the surface connection. On the tubing interior side of the gas-lift valve the pressure is determined by a number of factors, notably the static head of the fluid column above the valve, the flow rate of fluids up the tubing, the formation pressure, and the inflow rate in the oil production zone. Typically the orifice size of the gas lift valve is preset by selection at the time the valve is installed, and cannot be changed thereafter without changing the valve, which requires that the well be taken out of production.
The ongoing supply of compressed lift gas is a major determinant of production cost. The cost is a combination of the capital investment to provide the compressors and field infrastructure to convey the gas to each well, and the ongoing operating cost of running the compressors and maintaining them.
Many oil reservoirs have high-pressure gas caps or underlying high-pressure gas zones separated from the oil-bearing zones by impermeable layers. Nevertheless, in most situations the naturally-occurring reservoir gas is not used to lift the oil because of the inability to devise a method to monitor and control downhole operations. Attempts have been made to use reservoir gas for lift, see, e.g. U.S. Pat. Nos. 3,814,545 and 4,545,731, and Otis Engineering publication dated August 1980 entitled “Heavy Crude Lift Systems.” (Field Development Report OEC 5228, Otis Corporation, Dallas, Tex., 1980.) Instead, where it is necessary to provide a lift to the oil, a gas-lift well is used with compressed gas generated at the surface and forced downhole to lift the oil from the oil production zones. Hence, there is a need for a way to controllably use the naturally-occurring high-pressure gas already present downhole in one zone to provide gas lift for oil in another zone. An invention meeting this need may greatly increase the cost effectiveness of producing petroleum products using a gas-lift well.
Conventional packers are known such as described in U.S. Pat. Nos. 6,148,915, 6,123,148, 3,566,963 and 3,602,305.
All references cited herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be fully incorporated herein, it is incorporated by reference for background purposes, and indicative of the knowledge of one of ordinary skill in the art.